1. Field of the Invention
The present invention relates to an information recording and/or reproducing apparatus that optically records and/or reproduces information onto/from an information recording medium, such as an optical disc.
2. Description of the Related Art
Disc media, such as optical discs (hereafter referred to as “discs”), have been developed as leading information recording media because of their portability, usability, fast and random access/response characteristics, and the like. In recent years, with increases in packing densities and transfer rates of discs, small-diameter discs having large capacities have been employed in mobile image-recording apparatus, such as high-definition (HD) digital cameras and video camcorders. The market for such discs is growing.
In particular, HD-image-enabled products, such as compact-and-light HD cameras and HD displays, are rapidly becoming popular. Now, HD images, which once were handled only by commercial apparatuses, can be filmed or viewed with consumer apparatuses. Under these circumstances, to meet the market needs for filming and storing lengthy HD images, large-capacity discs are being developed.
One of the promising technologies to provide large-capacity discs is a multilayer technology in which a plurality of recording/reproducing layers are stacked in one disc. For example, the standardized number of recording/reproducing layers for digital versatile discs (DVDs) is two. A DVD has, from a side on which a light beam for recording and/or reproducing information is incident, a cover layer, a first recording layer, and a second recording layer. A reflective film is formed on the second recording layer, and an intermediate layer is provided between the first and second recording layers. Light quantities reflected from the first and second recording layers are designed to be equal.
A typical optical disc apparatus has an optical head (hereafter, referred to as an “optical pickup”) that emits a light beam to perform recording and/or reproducing of information onto/from a disc. The optical pickup consists of a laser source, an optical lens group including an objective lens for focusing a light beam onto a disc surface, an actuator for driving the objective lens, and the like.
The optical disc apparatus has a focus servomechanism and a tracking servomechanism for causing a beam spot to follow a target track on a disc surface in order to drive and to control the optical pickup and to perform recording and/or reproducing of information onto/from a disc. The tracking servomechanism detects light reflected from a disc with an optical sensor, processes the detection result, and generates a tracking-error signal. Radial-position control of the objective lens with respect to the disc is performed according to the polarity or level of the error signal.
A sled mechanism for radially moving the entirety of the optical pickup with respect to the disc is provided so that the objective lens moves in the vicinity of the optical axis of the optical pickup. The sled mechanism controls the optical pickup to access a desired track positioned from the center to the edge of the disc through operations such as address-seek, track-trace, and track-jump.
Multilayer-disc-enabled optical disc apparatuses have many problems. A variation in thickness of a layer between a light incident surface and each of the recording/reproducing layers of a disc causes wave aberration. Because the quality of the beam spot directly affects recording and/or reproducing of information, an increase in wave aberration degrades recording/reproducing quality, i.e., narrows a recording/reproducing margin.
Further, light reflected from a layer other than a tracking servo-access layer may leak into the tracking servo-access layer and produce a noise element (hereafter, referred to as “crosstalk”). This also narrows the recording/reproducing margin. These are inherent problems of multilayer discs.
Japanese Patent Laid-Open Nos. 10-106012, 08-212611, and 11-016200 disclose techniques to overcome these problems.
Japanese Patent Laid Open No. 10-106012 discloses a method of compensating for spherical aberration due to a variation in thickness of a disc, in which means for driving an optical element constituting an optical pickup in the optical axis direction is provided. Japanese Patent Laid-Open No. 08-212611 discloses an apparatus for compensating for spherical aberration, in which liquid crystal elements are provided in an optical path of an optical pickup. By applying a voltage to the liquid crystal elements, depending on the aberration, the phase of a light beam is differentiated, whereby compensation can be made for the spherical aberration.
Japanese Patent Laid-Open No. 11-016200 discloses an optical pickup in which an optical member for splitting reflected light into a plurality of rays is provided. By processing these rays, leakage of reflected light into a layer from another layer can be minimized, whereby a certain recording/reproducing margin can be obtained.
FIG. 14 is a qualitative diagram showing an increase in spherical aberration, which is a dominant aberration, in relation to an increase in the number of recording layers of a disc. In FIG. 14, the spherical aberration is plotted on the ordinate and the number of recording layers of a disc on the abscissa. As is well known, spherical aberration increases in proportion to an increase in variation in thickness of a disc. Accordingly, an increase in the number of recording layers constituting a disc, for example, from one layer to two layers, increases the spherical aberration, and consequently, degrades the quality of the beam spot. It is also known that an increase in numerical aperture (NA) of an objective lens of an optical pickup and use of a short-wavelength laser increase the spherical aberration. In particular, because of the multilayer structure of a disc, a farther recording layer (i.e., a recording layer farther from a light incident side) tends to be more largely affected by a variation in thickness of an intermediate layer and tends to have narrower margins, such as a recording/reproducing margin and a tilt margin.
Further, coma aberration caused by a tilt tends to occur due to factors, such as a disc having a multilayer structure, an objective lens having a high NA, and a laser having a short wavelength. Therefore, a farther recording layer has a narrower tilt margin. Thus, it is difficult for a multilayer-disc-enabled optical disc apparatus to obtain a recording/reproducing margin in a farther recording layer of a disc.
The methods and apparatuses described in the aforementioned Japanese Patent Laid-Open Nos. 10-106012, 08-212611, and 11-016200 intend to obtain a recording/reproducing margin by providing mechanisms that compensate for increases in spherical aberration.
Japanese Patent Laid-Open No. 10-106012 discloses a method in which means for driving an optical element is provided on an optical pickup for performing recording and/or reproducing of information onto/from a disc. Japanese Patent Laid-Open No. 08-212611 discloses an apparatus in which liquid crystal elements are provided. Japanese Patent Laid-Open No. 11-016200 discloses an optical pickup in which an optical member for splitting reflected light into a plurality of rays is provided. The methods and apparatuses described in the aforementioned three documents compensate for increased aberration by providing dedicated compensation mechanisms.
However, optical disc apparatuses for mobile use are assumed to be held by users with one hand and used outdoors. To make housings of these optical disc apparatuses compact, components constituting optical pickups are being further downsized. In addition, in order to meet the market needs for filming and storing lengthy HD images, large-capacity multilayer discs need to be developed. Obtaining a recording/reproducing margin in an optical disc is also extremely important.
The methods and apparatuses described in the aforementioned three documents have increased the size and weight of the optical pickups, the number of control signal lines, etc., and have failed to reduce the size and weight, as well as the manufacturing cost, of optical disc apparatuses.